In the mining, cleaning and transporting of coal and other minerals, considerable quantities of water and fine coal and minerals are processed. For example, long wall coal mining, which uses large quantities of water, produces wet coal which is finer and dirtier than coal and minerals produced by other processes. Additionally, coal or mineral cleaning frequently includes water washing, crushing and froth flotation which results in a coal/water mixture commonly referred to as a slurry. Such slurries are usually subjected to dewatering so that they may be shipped more economically, further processed more readily, or in the case of coal provide higher heat output per ton. In dewatering, the coal or mineral concentrate is slurry form is subjected to sedimentation, decantation, filtration, centrifugation etc., to facilitate the removal of water therefrom. A single mineral processing plant may employ a variety of individual dewatering unit operations. The output of these individual unit operations is typically blended to provide mineral i.e. coal of a desired specification of moisture and ash content.
The dewatering of a fine coal slurry is typically done by centrifuging. Centrifuging an aqueous coal slurry can produce fine coal having a water content of approximately 15 to 20% by weight. The use of certain surfactants as dewatering aids can improve the dewatering of fine coal being centrifuged. For example, small amounts of dioctylsulfosuccinate mixed with a fine coal slurry can produce a dryer product than obtained without application of the surfactant. However, the use of surfactants is not completely satisfactory. The surfactant can cause foaming of the slurry. Additionally, over time the surfactant can build up in the water which is recycled for reuse causing foaming problems.
In U.S. Pat. No. 4,290,897 a process for dewatering fine coal is described in which a water soluble organopolysiloxane plus a water emulsifiable organopolysiloxane is employed as a dewatering aid. In U.S. Pat. No. 4,853,133 a process for dewatering coal is described in which an ionic surfactant is employed as a dewatering aid in coal which is to be centrifuged and a foam suppressing amount of a cationic organic compound is added to the effluent. The cationic compound controls foaming by reacting with the ionic surfactant to form a solid precipitate which must be removed from the recycled water.
In the dewatering of fine coal or minerals by centrifuge or filtration, the formation of slimes by clay or shale material finer than -100 mesh and often -325 mesh can significantly decrease the permeability of the bed in the centrifuge or on the filter. U.S. Pat. No. 4,207,186 notes that in the removal of water from mineral concentrates by filtration the filter cake can often become substantially impermeable and significantly reduce filtration speeds. The patent teaches a dewatering aid combination that lowers the residual water content of the filter cake which comprises a hydrophobic alcohol and a nonionic surfactant.
Flocculants such as linear, long chain water soluble anionic or nonionic polymers based on polyacrylamide have been employed as filter aids. Flocculants bridge individual fine particles giving multi-particle aggregates which have a greater permeability and allow water to flow through the cake more readily.
In addition to the effects additives have upon the dewatering operation, the effects such additives have on water recycling are of great concern. The large volumes of water employed in such operations are recycled for economic and environmental reasons. The effluent water from a dewatering operation is typically clarified as in a settling pond where flocculants are routinely employed to remove clay. Any treatments added upstream, that is prior to dewatering, are necessarily selected so as not to adversely effect the clarification operations. For this reason, many potential upstream additives such as dispersants are generally avoided due to the belief that they would adversely effect the action of flocculants in the downstream clarifiers.